In recent years, a disk drive, such as a magnetic disk drive using a magnetic head for perpendicular magnetic recording has been proposed in order to achieve higher recording density, larger capacity, and miniaturization. In such a magnetic head, the recording head comprises a main pole configured to produce a perpendicular magnetic field and a write shield magnetic pole arranged on a trailing side of the main pole while interposing a write gap therebetween. Further, a recording head of a high-frequency assist recording mode has been proposed, in which a high-frequency assist element, for example, a spin torque oscillator, is provided in the write gap between the write shield magnetic pole and the main pole, and a high-frequency magnetic field is applied to the magnetic recording layer of a magnetic disk by the spin torque oscillator.
The above-described spin torque oscillator comprises a spin injection layer and an oscillation layer, which are arranged in the write gap. Usually, in a write gap, the gap magnetic field in a region (depth-side position) perpendicularly away from an air bearing surface (ABS) is stronger rather than that near the ABS. In other words, the magnetic field is non-uniform in the height direction of the write gap (in the depth direction from the ABS). Consequently, the magnetization of the oscillation layer and the spin injection layer is easily directed in the head traveling direction in the depth-side position away from the ABS, but on the ABS side, it becomes easily unstable. That is, it becomes difficult to rotate the magnetization of the oscillation layer uniformly. As a result, the spin torque oscillator cannot be made to oscillate excellently, degrading the oscillation characteristics.